2 Stage HPR Design Questions

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micro

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Getting started on a 2 stage, probably G to F motors. 2.5" diameter.

Wondering what is good practice or what people have done for locating the rail buttons. Should they be only on one stage or both? I'm accustomed to a single stage having an upper button near the CG and a lower button at the butt end. However, on this preliminary design my initial CG is in the upper stage.

Not sure if there's a pitfall that I don't see with having buttons on both sections? I can't imagine it somehow twisting and locking on the rail.

Also, for simplicity sake I want to use friction fit and upper stage thrust for separation. Any reason why I should reconsider shear pins and separation charge? Both stages will have their own altimeters and recovery devices.

Advice from those with experience greatly appreciated!
 
If rail buttons are used, putting them only on the booster is OK. The first pitfall that comes to my mind if using rail buttons located on both the booster and sustainer, would be extended unnecessary added drag. To clarify, if the buttons are only located on the booster then after booster burnout/sustainer separation, you no longer have any remaining drag due to the rail buttons.cHowever this may not be a concern at all for you (as I take it you are not going for performance) and this may require you to make modifications to your current design.

It sounds like you are planning on igniting the sustainer while on top of the booster, that just doesn't sound like the best route. How do you plan on igniting the sustainer? Depending on your design, the sustainer can have the tendency to drag separate and thus you can use a BP charge to aid the drag in separation of the sustainer. Doing so will require capable electronics to delay sustainer ignition until desired and would need to be housed inside the sustainer. Also I do not see a need for shear pins if you have a snug fit.
 
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OK... coming from my 1 2 stage flight...

I only used buttons on the booster. Reason being, you load the booster on the pad normally, then you arm the altims in the booster. Now, you put the sustainer on top of the booster, without having to feed rail buttons through on the rail, because I have live charges in my booster.


For separation, try putting a timer in the booster. Just a dumb and easy timer that operates off a G-switch. Make it fire a separation charge as close to after burnout as you can get it. This charge doesn't have to be massive, but a larger charge will ensure that the booster gets away from the sustainer as quickly as possible. I used 1/2 gram charge, and the booster had no issues separating.
 
I think there's lots of options here. If you're not concerned about maximum altitude, and if you are concerned about the sustainer going "up" should it light on the pad, then buttons can be placed on both stages (I put them about 8 inches apart near the bottom). With the typical (smaller) rail buttons, there is enough play in the buttons so that alignment won't be a problem if you're careful. I don't normally put buttons on the sustainer, but I do have a third button near the top of the booster on one rocket and a single button on the sustainer on another. These upper buttons are to keep the rocket stable and connected to the rail during set-up.

I don't want to have the sustainer motor come up to pressure while "contained" in the booster airframe. This might be OK, though, for smaller motors. So, I use a separation charge, but if the rocket drag separates, that's fine. If you have separate electronics in both sections, then there is no need to pin the stages together. However, you do want to pin the "nose cone" on the top of the booster so that it doesn't drag separate.

It is good form on two-stagers to use a properly configured shunt and/or switch to prevent the sustainer motor from lighting prematurely. It's the last thing you open and/or close before leaving the pad. It's also good form to use an altimeter that allows an altitude check before lighting the sustainer.

Jim
 
Thanks for the input, guys. I will go ahead and plan on buttons on both stages then. I'm really not concerned with performance other than safe flight. I'm a low/slow kind of guy anyway.

I'm still going to go with motor thrust separation for now though. It works great with my airstarted black powder motors and I'll only be going up to an F motor. In this case though I will add some small vents kind of like with gap staging for low power so the motor can pressurize without restriction.

Next question, where did you locate recovery in the booster? I'm designing mine so far with an e-bay/interstage coupler assembly as one piece at the top of the booster.
 
Thanks for the input, guys. I will go ahead and plan on buttons on both stages then. I'm really not concerned with performance other than safe flight. I'm a low/slow kind of guy anyway.

I'm still going to go with motor thrust separation for now though. It works great with my airstarted black powder motors and I'll only be going up to an F motor. In this case though I will add some small vents kind of like with gap staging for low power so the motor can pressurize without restriction.

Next question, where did you locate recovery in the booster? I'm designing mine so far with an e-bay/interstage coupler assembly as one piece at the top of the booster.

That's pretty much how it's done. Booster electronix in the ISC.

Fair warning tho... If you ignite the sustainer motor while it's still seated on top, you'll roast the daylights out of the ISC.

Got that tee shirt.
 
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Micro,

I don't post very often, but I don't think you have enough guidance from the community to safely start on your endeavor. Composite 2-stage rockets are exponentially more complex than single stage rockets. 2-stage rockets adhere to the equation of 1 + 1 = 5 in terms of complexity. That should not dissuade you, only raise your awareness. Pay attention to Jim's post. He is one of the VERY TOP multi-stage amateur rocketeers in the community. I know him personally and his stuff is amazing (Hi Jim - I miss flying with you and the guys at WTR). However, he mostly flies the BIG stuff. I fly much smaller 2-stage rockets, mostly I-to-H or J-to-I configurations. In 7 years I have flown 39 HPR 2-stage flights with an 80% success rate. Below are my thoughts on your proposal, but I welcome any criticism or suggestions for improvement from the community:

1) Small rockets do not require rail guides on the sustainer. I recommend using guides only on the booster.
2) Do you have HPR Level 1 certification? Almost any combination of G to F motors is in the H range. You will need HPR level 1 to fly your rocket in this configuration.
3) G-to-F is likely to be under-powered for any 2.5 inch rocket. Be sure to sim this configuration with actual weights and conservative Cd's. The most dangerous scenario is correct staging while pointed at the ground. If you are lighting the sustainer at less than 200 mph, then consider a larger booster motor. If you are adamant on using a G in the booster, then consider reducing your rocket diameter.
4) For Level 1 motors and less, a separation charge is not needed. Yes, the ISC will get burned a little, but I've done it over 30 times - no problem. Glassing the inside of the ISC will help counter any loss of structural integrity from burning.
5) While good practice, an altimeter to "check" vertical alignment is not necessary in smaller, subsonic rockets. If you sim it correctly, with a sufficiently sized booster motor, you will be fine.
6) Never, ever, ever use a low thrust motor in the booster. I've seen folks try it and it never turns out pretty. The booster should be a "fast" blue or white motor. No smokey's or sparky's. The wind will turn you sideways and you will light the sustainer while parallel to the ground. Cruise missile.
7) Electronic Dual Deployment in the sustainer is an absolute must. Without it you will lawn dart if the sustainer fails to light. This saved several of my rockets.
8) I use a perfect flight mini timer with a 7.4V 300mah 10C LiPo battery in the ISC to light the sustainer. This configuration will easily light even aerotech firstfire igniters.

Let me know if you have questions. We are here to help!

Sparky

Bryan Sparkman
Tripoli #12111, L3
NAR #85720, L3
 
Hey Bryan! How are things going?

Your post has lots of great advice. If I may expand just a bit ...

Composite 2-stage rockets are exponentially more complex than single stage rockets. 2-stage rockets adhere to the equation of 1 + 1 = 5 in terms of complexity.

2-Stage rockets are also more dangerous. I know two Tripoli board members that won't fly them and I know several people that were either injured or had close calls. F & G motors/rockets are large enough to get your attention if they light while you're next to them or fly in your direction under thrust. They deserve extra emphasis on safety. The most important things, IMO, are a strong ISC, an ignitor shunt and/or switch, an all-up test of the electonics just before going to the pad, a good simulation of the flight, and an altimeter or approach that can inhibit the motor if the flight isn't nominal. I have also taken to moving folks away from the rocket when I arm the electronics. The attached pic is the view when I do this.

I miss flying with you and the guys at WTR

I don't know if you've heard, but we're losing the field (to a solar farm). Jay and Larry are working on finding a new site.

In 7 years I have flown 39 HPR 2-stage flights with an 80% success rate.

You are not trying hard enough!

G-to-F is likely to be under-powered for any 2.5 inch rocket. Be sure to sim this configuration with actual weights and conservative Cd's.

I look at acceleration off the pad for selecting a booster motor. I've done flights with as little as 6 G's, but I'd recommend something between 10 - 15. Vmax motors in the booster are fine as long as the booster has electronic deployment.

For Level 1 motors and less, a separation charge is not needed.

I used to recommend using a separation charge to control the flight profile (i.e., the amount of coast before lighting the sustainer. However, I've come to realize that the two parts coasting together is fine, too. You just want to be able to control the timing.

While good practice, an altimeter to "check" vertical alignment is not necessary in smaller, subsonic rockets.

Not sure I agree here. It's not easy to use an altimeter to actually check the angle of the rocket, unless you know exactly how the altimeter works (the internal timing, how the data are filtered, etc.). The point of the check is to make sure that the sustainer won't light if something fails on the boost. If you expect the rocket to be at 2,000 feet at the point where you fire the sustainer ignitor, then including an altitude check at 1,500 feet, for example, means that the rocket is away from spectators and that it is likely still pointed up. There are a number of commercial altimeters that can do this check.

Electronic Dual Deployment in the sustainer is an absolute must.

Yep.

Jim

20140920_105627.jpg
 
I want to correct one point Bryan mentioned above.

You do not have to be certified to launch a complex rocket with 2 G motors provided that the total propellant weight does not exceed 125 grams, and the pad weight of the rocket does not exceed 1500 grams. No single motor may have a total impulse greater than 160 Ns nor can the average thrust exceed 80 N. If you are under 18, no single motor may contain more than 62.5 grams of propellant.

Reference: https://www.nar.org/wp-content/uploads/2014/08/MRSafetyCode.pdf

Bob
 
Thanks Bob - good catch! I stand corrected! I never knew that about 2X G motors. That's pretty neat and it opens Mod-Roc only guys to fly a little in the HPR range without the pain of certifying. 2-motor clusters are probably more common than 2-stagers though.

Jim, thats a shame about the field. I hope they get a new site soon as I want to come back and fly. I've been deployed overseas for 6-months and would like to fly at WTR when I get back. They recently evacuated me out of Yemen and I'm finishing my tour in Tampa, Fl - I'm broken hearted about it! (obviously I'm kidding).

I'm only aware of the Raven3 that has the altitude check prior to lighting the sustainer. What other ones are available? Its over $100 more expensive to use the Raven3 as compared to the Perfectflite mini-timer.

As for my 2-stage track record, most of my "failures" were due to failed sustainer ignition. I spent a long time tinkering with the electronics to minimize weight but maximize power. All but 1 recovery was nominal - one time my booster came in ballistic due to Quantum Tube + phenolic coupler + cold weather. Only once did it get hairy, when I forgot to dial-in a mach delay and the baro-based altimeter got fooled by the staging event. The sustainer lit at the same time the drogue chute deployed, resulting in skywriting at 1100 ft. A Raven3 could have prevented this.

Bryan
 
I'm only aware of the Raven3 that has the altitude check prior to lighting the sustainer. What other ones are available? Its over $100 more expensive to use the Raven3 as compared to the Perfectflite mini-timer.

The ones I'm aware of are the Raven 2 or Raven 3, the Eggtimer, the RRC3, the Telemetrum line, and the GWiz HCX. There may be more.

Jim
 
Thanks Bob - good catch! I stand corrected! I never knew that about 2X G motors. That's pretty neat and it opens Mod-Roc only guys to fly a little in the HPR range without the pain of certifying. 2-motor clusters are probably more common than 2-stagers though.

Jim, thats a shame about the field. I hope they get a new site soon as I want to come back and fly. I've been deployed overseas for 6-months and would like to fly at WTR when I get back. They recently evacuated me out of Yemen and I'm finishing my tour in Tampa, Fl - I'm broken hearted about it! (obviously I'm kidding).

I'm only aware of the Raven3 that has the altitude check prior to lighting the sustainer. What other ones are available? Its over $100 more expensive to use the Raven3 as compared to the Perfectflite mini-timer.

As for my 2-stage track record, most of my "failures" were due to failed sustainer ignition. I spent a long time tinkering with the electronics to minimize weight but maximize power. All but 1 recovery was nominal - one time my booster came in ballistic due to Quantum Tube + phenolic coupler + cold weather. Only once did it get hairy, when I forgot to dial-in a mach delay and the baro-based altimeter got fooled by the staging event. The sustainer lit at the same time the drogue chute deployed, resulting in skywriting at 1100 ft. A Raven3 could have prevented this.

Bryan

I would only ever use the Perfectflite mini-timer for lighting motors in clusters, not staging, and then only if the motors it's lighting are small compared to the central motor.

If you're expecting to be able to cheap out on the most complex type of flight, then you need to re-evaluate. Safety should always be the first priority.
 
Jim, do you have any more detail on how you configure the seperation charge? I've wondered about that charge disrupting the sustainer motor igniter.
 
Jim, do you have any more detail on how you configure the seperation charge? I've wondered about that charge disrupting the sustainer motor igniter.

I usually run two wires down the side of the motor - one for motor ignition and the other for the separation charge. For a 4" rocket, the charge might be 0.5 grams, contained in masking tape or whatever. I usually construct some sort of cover for the nozzle (which reminds me that I need to build one now). The type depends on the motor type. For AMW, it's a disc covering the bottom of the case. For CTI, a cap covering the nozzle. These just restrict the separation charge gases from impinging directly on the motor or the grains, but the caps aren't gas tight. I try to make sure the wires that are doing the motor ignition are reinforced in some manner so that they don't get damaged when the separation charge fires.

Jim
 
One comment. I witnessed the "reversed" logic one time of a K to L flight and it was pretty neat. K fired and staging to the L occurred relatively low so the L firing was easily seen.
Nice effect. Another advantage is if the sustainer motor doesn't fire and the electronics go into recovery mode, one doesn't have as far to walk to recover the rocket.
Of course one needs to be certain the chosen K is sufficient to safely move the stack but the effect is interesting with the relatively "down low" staging.
Kurt

Ooops, I just remembered. Give credit where credit is due. The flight I witnessed was by Fred Travertini (sp?)
 
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Hi Jim- can you provide some more commentary on each of these images?

The first two pics are the same cap. As I recall, I made the cap and then got worried the charge might cause it to break into the motor. So, I filled the center of it with epoxy up into the nozzle so that wouldn't happen.

The third is a cap on an AMW motor. Essentially like an altimeter bay cap with a lip. It just presses into the back of the motor case. The pic shows the two sets of "Gecko" wires - one for the separation charge and other for the motor igniter.

The fifth (right) is a cap that fits over the nozzle. It just has a gap cut into it for the wires. I believe it's just a small section of tubing with poured epoxy for the cover. I'll probably make another one of these.

Jim
 
I completely forgot about this thread but received an email that it was getting responses. I've spent the last couple years doing a lot of staging! Not always successful but always safe.

The first high power "style" was actually built out of an Estes Argent plus some extra parts. I often fly this one G to G but have flown it G76 to H54 once for my highest altitude flight to date, around 3,500'. I built it with a 38mm mount in the booster so I hope to fly it H to H some day, weather and field permitting.

FB_IMG_1439174490377.jpg

This one was built with a Perfectflight MiniTimer4 to light the sustainer, a Perfectflight StratologgerCF in the sustainer for dual deployment with motor backup, and a Missile Works RRC2+ in the booster's interstage coupler for apogee deployment with motor backup. The stages are slip-fit, motor separation. I did use two rail buttons on each section. Here is a vid of its second flight:

https://youtu.be/A3db-rROp8M

I have had two failures with this, both safely recovered. Both were Cesaroni motors oddly enough, I've had perfect track record airstarting Aerotechs. One lit the ematch which burned the black powder pellet which separated the stages but failed to ignite the propellant. Both stages recovered successfully with their altimeters. The other failure was a bad ematch, it had continuity on the pad, the MiniTimer4 fired it at the appropriate time, but it popped the bridge wire without igniting the pyrogen! Post flight the ematch looked new but had no continuity. So that can happen too! This one was a little more precarious since the stages remained together and both altimeters popped chutes at apogee, so everything came down under chute albeit a tangled mess!

After a handful of successful flights on that I built an upscale Comanche 3. Similarly built, 38mm to 29mm to 29mm, buttons on all three stages. Slip fit, motor separation. I've always had perfect flights with this model. This posed some design challenges in a 2.5" airframe due to the short boosters, but I got it done. First booster is simple motor ejection, second stage lights the motor and ejects a chute at apogee with a Featherweight Raven3, and the sustainer lights with a Raven3 and has dual deployment with a StratologgerCF. This flies to about 3000' on H123 to G64 to G64.

Comanche-3-Frame.jpg

Turning up three altimeters on the pad is entertaining but my ears are still good :cool:

And a video: https://youtu.be/5o9QKJjwDcI
 
As for separating stages and protecting the igniter, I use about a half gram of BP in a centrifuge vial at the bottom of the interstage. Next goes a small scrap of Kevlar or Nomex tethered to the lower stage. Has worked dozens of times without fail. Ksaves2, what flight did you see?
 
Great video on the Comanche..
Interesting how that 1st booster stage started to rotate right before the 2nd stage ignition.

So if I understand correctly - these are not exactly drag separation projects - but "slip fit, motor separation"....in other words, the stages slip fit together like a drag separation project (but no shear pins used), but it is the motor ignition (not a BP charge) for the staging...?
 
Good looking rockets!

Thanks!

Great video on the Comanche..
Interesting how that 1st booster stage started to rotate right before the 2nd stage ignition.

So if I understand correctly - these are not exactly drag separation projects - but "slip fit, motor separation"....in other words, the stages slip fit together like a drag separation project (but no shear pins used), but it is the motor ignition (not a BP charge) for the staging...?

Correct. They would be drag separated but at the velocity I'm going there isn't enough drag to separate them. That's why I purposely said "slip fit" instead of "drag separated." I calculated the drag force to be very small, like 1 or 2 oz on each booster. The connection between stages 2 and 3 is a bit more snug like a nose cone but the connection between stages 1 and 2 is more loose fitting. I didn't want the drag of stage 1 to pull stage 2 off with it. But that's why you see it start to move, it's pretty loose but there just isn't quite enough drag to pull it off. I did consider the possibility of stage 1 pulling off stage 2 with it, that's one reason why stage 2 has its own altimeter. The boosters are so short they'd probably just tumble but you never know.

BTW one advantage to having rail buttons on all 3 stages was I was able to fly shakedown flights of sustainer only and 2 stage. That way I was able to use the flight data to dial in the actual drag forces. Also because of the short boosters, the static ports for stage 2's altimeter had to go between fins, so flying two stage with motor ejection and ridealong altimeter in the booster allowed me to verify those static ports between fins would work fine.
 
Nice flights. Just to be clear - exactly what igniters did you use on your sustainer(s)? I am getting ready to try something similar with the MiniTimer4 and a tiny 7.4V lipo that easily lights 4 e-matches. Those don't quite fit like I would like in the Aerotech 29mm reload. Did you just use the lighter that came with the motors? The MiniTimer4 manual specifically says to watch out about using the FirstFire (I guess it is a Jr.) that comes with it.
 
Im considering a single rail button in my 12" booster. The sustainer is a 3" Osprey that has 2 buttons on it. When using a short booster section should I put 2 buttons on it or leave them off the booster?
 
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